An FBI agent dubbed the “modern-day Donnie Brasco” revealed fascinating details yesterday about how he secretly infiltrated and lived among the Gambinos – including once dining with an acting capo and then-married power couple Tommy Mottola and Mariah Carey.

The now-retired agent, testifying in a closed Manhattan federal courtroom, described how he amazingly tricked the Mafia crime family into believing he was goodfella wannabe “Jack Falcone.”

The former fed even revealed his true identity – Cuban-born Joaquin Manuel Garcia.

“I took the role of being Italian, fourth or fifth generation removed from Sicily,” Garcia told the jury, as reporters listened to relayed testimony in a separate room. “The ruse was I was a knock-around guy who had access to everything.”

Armed with thousands of hours of secretly recorded tapes, Garcia testified that it was Greg DePalma – a 74-year-old reputed acting capo on trial for racketeering – who took him under his wing and gave him an earful on the inner workings of the Gambinos.

The agent described DePalma’s alleged extortion plots in colorful detail, including the shakedown of Valbella restaurant, a five-star eatery in Greenwich, Conn., where the accused mob big once dined with Sony’s Mottola and the music honcho’s then-wife, pop star Mariah Carey.

The undercover, who posed as a fence and strip-club investor with ties to Cuban gangs in Miami, played a key role in last year’s roundup of 32 reputed gangsters, including much of the Gambino hierarchy.

Garcia recounted how DePalma – who’s been compared to the real life Brasco played by Johnny Depp in the 1997 movie – was wary when they first met at a Bronx strip club in early 2003. The agent gained the capo’s trust by offering him counterfeit cigarettes to sell.

“He was always with a cigarette in his mouth. They were always Camel, nonfilter,” Garcia said of DePalma, who lost a lung to cancer and breathes oxygen through a tube in his nose while he’s in court.

The agent said he followed up by supplying DePalma with “stolen” jewelry, Rolex watches and plasma TVs.

By June of that year, DePalma was so convinced by the agent’s performance as Falcone, he offered to put the agent “on record” with his crew during a Westchester meeting that was secretly recorded outside of La Gravinese Jewelry Store in Pelham.

The gravelly voiced DePalma was recorded telling Garcia, “You never been with any wiseguy before, did you . . . Well, I put you on record with me, with my family. Uh?”

“It’s an honor,” the agent replied.

“Nobody could bother you. Nobody could come near you, no wiseguys, nothing. I don’t care if they’re the boss, the underboss of another crew,” DePalma allegedly said.

The agent said he and DePalma spoke on the phone every day for two years and met in person up to five times a week – interactions that were secretly recorded.

On Oct. 6, 2003, Garcia taped DePalma detailing how he controlled Valbella and dined there for free, once with Mottola and Carey in 1999.

On the tape, DePalma described Carey as “very nice, very quiet, reserved.”

DePalma went on to describe spotting Gambino underboss Anthony Megale seated in the back room at a table of 16 and called over the owner.

“I said, ‘See that table of 16? No check,’ ” DePalma said.

“Oh, man,” said the undercover agent, laughing.

DePalma continued, “He’s got a wine cellar. Maybe it’s worth $4 or $5 million. I said, ‘Go down there and bring that $11,000 bottle of wine up here. Chateau Lafitte.”

DePalma also boasted that he forced the restaurateur to give him seven bottles of equally expensive wine for his doctor.

In another shakedown scheme in October 2003, the agent said DePalma forced Liza Minnelli’s former manager Gary Labriola to shell out $12,000 so reputed acting boss Arnold Squitieri’s wife, sister and pals could enjoy a luxury vacation in Las Vegas.

In another tape from October 2003, DePalma bemoans the state of the Mafia to Garcia.

“This life in Italy, they were supposed to rob from the rich and give it to the poor, like Robin Hood. Today, these c- – -suckers that they straighten out, the wiseguys, they’re robbing from the f- – -ing poor,” DePalma said.

This conversation took place between Greg DePalma (right) and undercover agent Jack Falcone on June 6, 2003, outside La Gravinese jewelry store in Pelham, N.Y.

GD: You never been with a wiseguy before, did you?

JF: No, only Cuban guys.

GD:Well, I put you on record with me, with my family. Uh?

JF: It’s an honor.

GD: You know what that means right? OK?

JF: Yeah.

GD: Nobody could bother you. Nobody could come near you, no wiseguys, nothing. I don’t care if they’re the boss,

the underboss of another crew. I couldn’t do it before because I did not have your last name until last week,